Ferritic stainless steel

ABSTRACT

A ferritic stainless steel is provided having a chemical composition consisting of, by mass %, C: 0.003% or more and 0.015% or less, Si: 0.05% or more and 0.30% or less, Mn: 0.10% or more and 0.35% or less, P: 0.06% or less, S: 0.02% or less, Cr: 17.0% or more and 19.0% or less, Ni: more than 0.10% and 0.30% or less, Ti: 0.10% or more and 0.40% or less, Nb: 0.005% or more and less than 0.050%, Mo: less than 0.20%, N: 0.005% or more and 0.015% or less, Cu: 0.30% or more and 0.50% or less, Mg: less than 0.0005% and the balance being Fe and inevitable impurities.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2013/001462, filedMar. 7, 2013, which claims priority to Japanese Patent Application No.2012-055308, filed Mar. 13, 2012, the disclosures of each of theseapplications being incorporated herein by reference in their entiretiesfor all purposes.

FIELD OF THE INVENTION

The present invention relates to a ferritic stainless steel excellent insurface quality, and excellent in corrosion resistance in a part weldedwith an austenitic stainless steel.

BACKGROUND OF THE INVENTION

Although, among stainless steels, SUS304 (18% Cr-8% Ni) (JapaneseIndustrial Standard, JIS G 4305), which is an austenitic stainlesssteel, is widely used because of its good corrosion resistance, thiskind of steel is expensive because a large amount of Ni is containedtherein. Therefore, the stainless steel according to Patent Literature 1was developed as a steel having good corrosion resistance equivalent tothat of SUS304.

Patent Literature 1 discloses a ferritic stainless steel having achemical composition consisting of, by massa, C: 0.03% or less, Si: 1.0%or less, Mn: 0.5% or less, P: 0.04% or less, S: 0.02% or less, Al: 0.1%or less, Cr: 20.5% or more and 22.5% or less, Cu: 0.3% or more and 0.8%or less, Ni: 1.0% or less, Ti: 4(C %+N %) or more and 0.35% or less, Nb:0.01% or less, N: 0.03% or less, C+N: 0.05% or less and the balancebeing Fe and inevitable impurities.

Ferritic stainless steels such as JIS-SUS444 and JIS-SUS430J1L arecharacterized by having low stress corrosion cracking sensitivity incomparison to austenitic stainless steels and not containing a largeamount of Ni which is subject to wide price fluctuations, and are widelyused as materials for exhaust system parts of automobiles, water tanksand building constructions.

However, since ferritic stainless steels have inferior formability,especially poor ductility, comparing to austenitic stainless steels,austenitic stainless steels are used for parts which are too difficultto be formed with ferritic stainless steels. Therefore, there are manycases where one component is formed by combining austenitic stainlesssteels and ferritic stainless steels. Among these cases, most parts arejoined together by welding, and, among welding methods, TIG welding(Tungsten Inert Gas welding) is mainly used. And good corrosionresistance is required for welded parts as well as base steels.

PATENT LITERATURE

PTL 1: Japanese Unexamined Patent Application Publication No. 2007-77496

PTL 2: Japanese Unexamined Patent Application Publication No. 8-10823

SUMMARY OF THE INVENTION

The ferritic stainless steel according to Patent Literature 1 has goodcorrosion resistance in a part welded with a stainless steel of the samekind. However, there is a problem in that the corrosion resistance in awelded part is inferior to that in a base steel in the case where theferritic stainless steel is welded with a stainless steel of thedifferent kind such as SUS304 by performing TIG welding.

This problem is caused as follows; C or N in a steel combines with Cr,precipitating chromium carbides such as Cr₂₃C₆ or chromium nitrides suchas CrN₂ at grain boundaries in the thermal history of welding. Theseprecipitations form chromium depletion layers, where Cr content is lessthan the base steel, in the vicinity of the grain boundaries. Thisphenomenon, called sensitization, causes deterioration in corrosionresistance at the grain boundaries.

Generally, in order to prevent deterioration in corrosion resistance ina welded part due to sensitization, the formation of chromium carbidesand chromium nitrides are prevented by adding an appropriate amount ofTi to steel and stabilizing C and N as titanium carbonitrides withdecreasing C and N contents in steel. By this method, the welded partwhich is formed by performing TIG welding using the same ferriticstainless steels according to, Patent Literature 1 has good corrosionresistance.

However, since SUS304 has C content of from 0.04% to 0.05%, which ismore than that of this ferritic stainless steel sheet whose C content isabout 0.01%, in order to similarly prevent sensitization by adding Ti inthe case where this ferritic stainless steel sheet is welded with a highcarbon stainless steel such as SUS304, it is necessary to increase Ticontent to about 1.0%.

However, in the case where the Ti content of ferritic stainless steel isincreased up to about 1.0%, Ti and N in molten steel react with eachother to form and precipitate TiN during solidification. Since TiN haslow ductility at a high temperature, it causes defects in a hot rollingprocess and deteriorates surface quality. Since the defects formed′ asdescribed above are too deep to be eliminated during subsequentprocesses such as annealing of hot-rolled steel sheet, pickling ofhot-rolled steel sheet, cold rolling of hot-rolled steel sheet,annealing of cold-rolled steel sheet and pickling of cold-rolled steelsheet. The defects become surface defects called stringers caused bytitanium nitrides, resulting in significant deterioration in surfacequality of the cold-rolled, annealed and pickled steel sheet, unlessperforming a treatment in which a thick layer is scraped off the surfaceof the hot-rolled, annealed and pickled steel sheet by a grinder or thelike.

In addition, although TIG welding is generally performed underconditions in which both front and back sides of steel sheet areshielded with an inert gas so that the formation of thin oxidized layercalled temper color is prevented as much as possible. Since, in apractical process, this gas shield is not sufficient, there is a problemin that sensitization described above is facilitated due to mixing of Nfrom air.

In addition, there is also a problem in that adding expensive Ti in alarge amount decreases the advantage of the ferritic stainless steelwhich does not use expensive Ni.

The present invention has been completed in view of the situationdescribed above, and an object of the present invention is to provide aferritic stainless steel excellent in surface quality, and corrosionresistance in a welded part in the case that the ferritic stainlesssteel is welded not only with a ferritic stainless steel but also withan austenitic stainless steel.

The present inventors conducted exhaustive experimentations andinvestigations not only on the influence of the chemical composition ofsteel on the corrosion resistance of base steel and welded part, butalso on the surface quality (stringer flaw caused by titanium nitrides)of steel sheet in order to solve the problems described above, and, as aresult, obtained the following findings.

(1) Sensitization can be prevented by adjusting the contents of ferriticformer elements and by making the microstructure of a part welded with aferritic stainless steel and an austenitic stainless steel a martensitephase. This is because the solubility limits of C and N are large in amartensite phase.

(2) By adding a very small amount of Nb, Nb nitrides are precipitated ata temperature higher than temperature at which Ti nitrides areprecipitated. In a cooling process thereafter, these Nb nitrides becomenucleation sites of Ti carbonitrides. This phenomenon enhances an effectof Ti to prevent sensitization.

(3) The crystallization temperature of steel sheet is generally raisedby adding Nb. Since there is almost no negative effect that thecrystallization temperature of steel sheet is raised when a very smallamount of Nb is added, an inexpensive high-speed pickling process usedin a manufacturing line of carbon steel disclosed in Patent Literature 2can be applied.

(4) Even if N in air is mixed in a welded part due to incomplete gasshield during welding, sensitization can be avoided by the formation ofAlN in the welded part in the case where an appropriate amount of Al isadded to steel. In addition, sensitization can be avoided due to theformation of compounds of Sb and N in a welded part in the case where anappropriate amount of Sb is added to steel.

(5) A stringer flaw caused by titanium nitrides is mainly caused by TiNof a large size existing in an outermost layer of steel sheet. Astringer flaw caused by titanium nitrides can be avoided by adjusting Ticontent.

From the findings described above, a ferritic stainless steel which hasexcellent corrosion resistance in a welded part and excellent surfacequality of a cold-rolled, annealed and pickled steel sheet withouthaving to grind the surface of a hot-rolled, annealed and pickled steelsheet and is less expensive than austenitic stainless steels containingNi, can be achieved.

The present invention has been completed on the basis of the findingsdescribed above and the present invention includes the following.

[1] A ferritic stainless steel having a chemical composition consistingof, by mass %, C: 0.003% or more and 0.015% or less, Si: 0.05% or moreand 0.30% or less, Mn: 0.10% or more and 0.35% or less, P: 0.06% orless, S: 0.02% or less, Cr: 17.0% or more and 19.0% or less, Ni: morethan 0.10% and 0.30% or less, Ti: 0.10% or more and 0.40% or less, Nb:0.005% or more and less than 0.050%, Mo: less than 0.20%, N: 0.005% ormore and 0.015% or less, Cu: 0.30% or more and 0.50% or less, Mg: lessthan 0.0005% and the balance being Fe and inevitable impurities.

[2] The ferritic stainless steel according to [1], the steel having achemical composition further containing, by mass %, Al: 0.02% or moreand 0.50% or less.

[3] The ferritic stainless steel according to [2], the steel having achemical composition containing, by massa, Al: 0.10% or more and 0.50%or less.

[4] The ferritic stainless steel according to any one of [1] to [3], thesteel having a chemical composition further containing, by mass %, Sb:0.005% or more and 0.300% or less.

[5] The ferritic stainless steel according to any one of [1] to [4], thesteel having a chemical composition further containing, by mass %, atleast one of Zr: 0.05% or more and 0.60% or less and V: 0.02% or moreand 0.50% or less.

The ferritic stainless steel can be ideally used as materials forkitchen instruments, architectural interiors, industrial machines andautomobile parts, because the ferritic stainless steel has excellentcorrosion resistance in a part welded even with an austenitic stainlesssteel and has excellent surface quality.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The preferred limitations on the factors constituting the presentinvention will be described hereafter.

1. Regarding a chemical composition The reason for the preferredlimitations on the chemical composition of steel according to thepresent invention will be described. Here, % used when describing achemical composition always means mass %.

C: 0.003% or more and 0.015% or less

It is preferable that C content be as small as possible, because C tendsto combine with Cr to form Cr carbides, and intergranular corrosion iscaused by Cr carbides which are formed in a heat affected zone duringwelding. Therefore, the C content is confined to be 0.015% or less. Onthe other hand, since a long time is necessary for smelting in the casewhere the C content is excessively small, the C content is confined tobe 0.003% or more and 0.015% or less, preferably 0.003% or more and0.012% or less from the viewpoint of corrosion resistance in a weldedpart, more preferably 0.003% or more and 0.010% or less.

Si: 0.05% or more and 0.30% or less

Since Si is a chemical element which is effective as a deoxidizingagent, the Si content is confined to be 0.05% or more. On the otherhand, in the case where the Si content is more than 0.30%, thehigh-speed pickling performance in a manufacturing line of carbon steelis decreased, which results in a decrease in productivity. Therefore,the Si content is confined to be 0.05% or more and 0.30% or less,preferably 0.05% or more and 0.20% or less.

Mn: 0.10% or more and 0.35% or less

Since Mn is effective for deoxidation, the Mn content is confined to be0.10% or more. In addition, since Mn is an austenite former element, Mnpromotes the formation of a martensite phase in a part welded with anaustenitic stainless steel (hereinafter, called the welded part ofdifferent steels). However, since, in the case where the Mn content isexcessively large, Mn combines with S in steel to form MnS which is asoluble sulfide, which results in a deterioration in corrosionresistance, the Mn content is confined to be 0.10% or more and 0.35% orless, preferably 0.10% or more and 0.25% or less.

P: 0.06% or less

In the case where P content is more than 0.06% P, P not only has anegative effect on corrosion resistance, but also deterioratesformability due to solid solution strengthening. Therefore, the Pcontent is confined to be 0.06% or less, preferably 0.04% or less fromthe viewpoint of corrosion resistance.

S: 0.02% or less

S is a chemical element which has a negative effect on corrosionresistance. In particular, in the case where S is present together withMn, S becomes a source of pitting as a result of forming MnS, resultingin deterioration in corrosion resistance. This negative effect becomessignificant in the case where S content is more than 0.02%. Therefore,the S content is confined to be 0.02% or less, preferably 0.01% or lessfrom the viewpoint of corrosion resistance, more preferably 0.006% orless.

Cr: 17.0% or more and 19.0% or less

Cr is a chemical element which is essential for increasing corrosionresistance of a base stainless steel by forming a passivation film onthe surface of steel. Cr content of 17.0% or more is necessary in orderto achieve good corrosion resistance. However, in the case where the Crcontent is more than 19.0%, deterioration in corrosion resistance in thewelded part with SUS304 cannot be prevented, because a martensite phaseis not formed in that part. Therefore, the Cr content is confined to be17.0% or more and 19.0% or less, preferably 17.5% or more and 18.5% orless.

Ni: more than 0.10% and 0.30% or less

Ni is a chemical element which contributes to improving crevicecorrosion resistance. Moreover, since Ni is an austenite former elementlike Mn, Ni promotes the formation of a martensite phase in the weldedpart of different steels. However, there is deterioration in StressCorrosion Cracking sensitivity in the case where Ni content is more than0.30%. In addition, Ni is an expensive chemical element. Therefore, theNi content is confined to be more than 0.10% and 0.30% or less,preferably 0.20% or more and 0.30% or less.

Ti: 0.10% or more and 0.40% or less

Ti is a chemical element which is essential for achieving good corrosionresistance in the welded part of different steels when it is welded withan austenitic stainless steel as described above. However, excessive Ticontent causes an increase in the amount of precipitated TiN, whichresults in a significant number of stringer flaws caused by titaniumnitrides, making it impossible to achieve good surface quality for aproduct (cold-rolled, annealed and pickled steel sheet) withoutperforming a treatment such as grinding the surface of a hot-rolled,annealed and pickled steel sheet. Therefore, the Ti content is confinedto be 0.10% or more and 0.40% or less, preferably 0.20% or more and0.40% or less from the viewpoint of corrosion resistance in the weldedpart of different steels.

Nb: 0.005% or more and less than 0.050%

Small Nb adding is also one of the important factors for the presentinvention. Nb forms carbonitrides more readily than Cr and Ti. Inparticular, in the case of the welded part of different steels, in weldmetal and a heat affected zone, the formation of Nb carbonitrides beginsat a temperature higher than the temperature at which Ti carbonitridesare formed. In a cooling process thereafter, although the reason forthis is not clear, the Nb carbonitrides become nucleation sites of Ticarbonitrides. That is to say, since small Nb content accelerates theformation of Ti carbonitrides, the ability of Ti to stabilize C and N inweld metal and a heat affected zone of the welded part of differentsteels becomes stronger in comparison to the case where Nb is notcontained, causing sensitization to be prevented more effectively.Therefore, the lower limit of the Nb content is confined to be 0.005% ormore. On the other hand, since excessive Nb content causes an increasein recrystallization temperature of a cold-rolled steel sheet, it isnecessary to anneal the steel sheet at a high temperature in order toachieve sufficient mechanical properties. This increases the thicknessof an oxidized layer, which is formed during finishing annealing, incomparison to the case where Nb is not contained. This deteriorates thepickling performance of a cold-rolled steel sheet in a high-speedpickling process which is used in a manufacturing line of carbon steeldescribed above, resulting in deterioration in productivity. Therefore,the Nb content is confined to be 0.005% or more and less than 0.050%,more preferably 0.010% or more and less than 0.050% from the viewpointof corrosion resistance in the welded part of different steels.

Mo: less than 0.20%

Mo strengthens a passivation film and significantly improves corrosionresistance. However, since Mo is a ferrite former element, even in thecase where Mo content is small, a martensite phase is not formed in thewelded part of different steels when it is welded with an austeniticstainless steel. This forms the welded part of different steelsconsisting of a ferrite phase and causes sensitization. Therefore, theMo content is confined to be less than 0.20%. In addition, since Mocauses deterioration in the toughness of a hot-rolled steel sheet, it ispreferable that the Mo content be less than 0.10%. Incidentally, thelower limit of the Mo content is confined to be 0%.

N: 0.005% or more and 0.015% or less

N easily combines with Cr to form Cr nitrides. It is preferable that Ncontent be as small as possible, because Cr nitrides cause intergranularcorrosion in the case where the Cr nitrides are formed in the weldedpart of different steels and in a heat affected zone when welding isperformed. In addition, it is preferable that the N content be as smallas possible in order to decrease the amount of precipitated TiN whichbecomes the source of stringer flaws caused by titanium nitrides.However, since smelting takes a long time in the case where the Ncontent is excessively reduced, the N content is confined to be 0.005%or more and 0.015% or less, preferably 0.005% or more and 0.012% or lessfrom the viewpoint of corrosion resistance in the welded part ofdifferent steels, more preferably 0.005% or more and 0.010% or less.

Cu: 0.30% or more and 0.50% or less

Cu is a chemical element which improves corrosion resistance, inparticular, in the case where a steel is placed in an aqueous solutionor covered with weakly acidic water drops. This is because Cuwhich′covers the surface of the base steel after dissolving in theaqueous solution or the water drops prevents the dissolution of the basesteel. However, in the case where Cu content is more than 0.50%, thereis deterioration in formability at a high temperature and surfacedefects are caused by jelly-like oxides which are called red scales andwhich are formed on the surface of hot-rolled steel sheet due to Cu.Therefore, the Cu content is confined to be 0.30% or more and 0.50% orless, preferably 0.30% or more and 0.40% or less from the viewpoint offormability at a high temperature.

Mg: less than 0.0005%

Mg is an impurity which is mixed in mainly from the bricks of converterfurnace. Mg becomes the source of various kinds of inclusion andnucleation sites of other kinds of inclusion. In addition, since theinclusions are less likely to be dissolved, even when performing atreatment such as annealing, Mg deteriorates the surface quality of notonly hot-rolled, annealed and pickled steel sheet and but also product(cold-rolled, annealed and pickled steel sheet). Therefore, the Mgcontent is confined to be less than 0.0005%, preferably less than0.0003%.

Although the basic chemical composition according to embodiments of thepresent invention is as described above and the balance consists of Feand inevitable impurities, Al and Sb may be further contained from theviewpoint of preventing sensitization of the welded part of differentsteels caused by insufficient gas shielding of TIG welding. Moreover, Zrand V may be contained in order to improving corrosion resistance of thewelded part of different steels. Here, examples of acceptable inevitableimpurities include Ca: 0.0020% or less but it is not necessary to limitto Ca.

Al: 0.02% or more and 0.50% or less

Al is a chemical element which is important in particular in the casewhere a gas shield for TIG welding is not satisfactory. Generally, theback side of steel sheet is shielded with gas when TIG welding isperformed as described above. However, in the case where the shape ofthe welded part of different steels is complicated, gas shield is notsufficiently effective and N in air may be mixed into weld metal. Inthis case, sensitization cannot be completely prevented through addingTi only, if the content of C and N is more than the solid solubilitylimit of martensite phase. In this case, it is effective to add Al inadvance to prevent sensitization. This is because Al stabilizes N inweld metal as AlN. This effect can be realized by confining Al contentto be 0.02% or more. However, in the case where the Al content is morethan 0.50%, non-metal inclusions are formed in a slab, which results indeterioration in surface quality of hot-rolled steel sheet andcold-rolled steel sheet. Therefore, in the case where Al is contained,it is preferable that the Al content be 0.02% or more and 0.50% or less.A more preferable lower limit of the Al content is 0.10%, further morepreferably 0.15%. A more preferable upper limit of the Al content is0.30%.

Sb: 0.005% or more and 0.30% or less

Sb is a chemical element which is better to add in the case where acomponent has a complicated shape, since Sb is, like Al, effective forstabilizing N mixed from air in the case where a gas shield of TIGwelding is not sufficiently effective. However, in the case where Sbcontent is excessively large, non-metal inclusions are formed in a slab,which results in deterioration in surface quality of hot-rolled steelsheet and cold-rolled steel sheet. Therefore, in the case where Sb iscontained, it is preferable that the Sb content be 0.005% or more and0.30% or less, more preferably 0.005% or more and 0.10% or less from theviewpoint of the surface quality of product (cold-rolled, annealed andpickled steel sheet).

Zr: 0.05% or more and 0.60% or less

Zr is a chemical element which is effective for improving corrosionresistance in the welded part not onlt of same steels but also ofdifferent steels by forming, like Ti, carbonitrides more readily thanCr. However, Zr is more expensive than Ti and, in the case where Zrcontent is excessively large, Zr forms intermetallic compounds, whichresults in deterioration in toughness of hot-rolled steel sheet.Therefore, in the case where Zr is contained, it is preferable that theZr content be 0.05% or more and 0.60% or less, more preferably 0.15% ormore and 0.35% or less.

V: 0.02% or more and 0.50% or less

V is also a chemical element which is effective for improving corrosionresistance in the welded part not only of same steels but also ofdifferent steels by forming, like Ti, carbonitrides more readily thanCr. However, this effect of V is smaller than that of Ti. Also, V isexpensive. Therefore, in the case where V is contained, it is preferablethat the V content be 0.02% or more and 0.50% or less, more preferably0.02% or more and 0.05% or less.

2. Regarding manufacturing conditions

Subsequently, the preferable method for manufacturing the steelaccording to the present invention will be described. Steel having achemical composition described above is smelted using a well-knownmethod such as a converter furnace, an electric furnace or a vacuummelting furnace, and the smelted steel is made into a steel material(slab) using a continuous casting method or an ingot casting-bloomingmethod. This steel material is heated at a temperature in the range from1100° C. to 1250° C. for a duration of from 1 hour to 24 hours and, ordirectly without heating, hot-rolled into a hot-rolled steel sheet.

Although the hot-rolled steel sheet is generally subjected to annealingat a temperature in the range from 800° C. to 1100° C. for a duration offrom 1 minute to 10 minutes, this annealing may be omitted depending onuse application. Then, after being subjected to pickling, the hot-rolledsteel sheet is cold-rolled into a cold-rolled steel sheet, and thecold-rolled steel sheet is made a product by performing finishingannealing. It is preferable that cold rolling is performed with arolling reduction ratio of 50% or more from the viewpoint of elongationperformance, bending performance, press forming performance and shapeleveling. It is preferable that finishing annealing of cold-rolled steelsheet be generally performed at a temperature in the range from 800° C.to 950° C. in the case of No. 2B finish in accordance with JIS G 0203.

However, in the case of manufacturing a product using tandem coldrolling line and continuous annealing line at a high productivity, it ismost preferable that the product be manufactured in an inexpensiveprocess using a high-speed pickling method (refer to Patent Literature2) of annealing and pickling line for carbon steel as described above,and, in this case, it is preferable that annealing temperature be in therange from 800° C. to 900° C. In addition, in the case of a product forparts in which luster is more required, finishing annealing using BAannealing method is effective. As described above, there is no problemin that a treatment such as polishing is performed after cold rolling orforming has been performed in order to achieve further better surfacequality.

Example 1

The present invention will be described more in detail in reference toexamples hereafter.

Steels having a chemical composition of examples No. 1 through No. 8 andNo. 33 and comparative examples No. 9 through No. 12 given in Table 1were smelted using a small vacuum melting furnace having a capacity of50 kg. The ingots of these steels were heated at a temperature of 1150°C. in a furnace under an Ar gas purge and hot-rolled into hot-rolledsteel sheets having a thickness of 4.0 mm.

Subsequently, these hot-rolled sheets were subjected to annealing in airat a temperature of 950° C. for a duration of 1 minute, followed by asurface treatment using shot blasting with glass beads, and descaling bypickling in which the steel sheets were dipped in a sulfuric acidsolution containing sulfuric acid in a concentration of 20 mass % at atemperature of 80° C. for a duration of 120 seconds and then in a mixedacid solution containing nitric acid in a concentration of 15 mass % andhydrofluoric acid in a concentration of 3 mass % at a temperature of 55°C. for a duration of 60 seconds.

Then, the hot-rolled steel sheets were cold-rolled into cold-rolledsteel sheets having a thickness of 1.0 mm, subjected to annealing in afurnace in air at a temperature of 900° C. for a duration of 1 minuteand made cold-rolled and annealed steel sheets. These cold-rolled andannealed steel sheets were subjected to electrolytic descaling with thesteel sheet being a positive electrode for three times in a solutioncontaining NaSO₄ in a concentration of 20 mass % at a temperature of 80°C. with a current of 3 A/dm² for a duration of 10 seconds, and descalingin a mixed acid solution containing nitric acid in a concentration of 5mass % and hydrofluoric acid in a concentration of 3 mass % at atemperature of 55° C. for a duration of 30 seconds in order to havecold-rolled, annealed and pickled steel sheets.

TABLE 1 Chemical Composition (mass %) No. C Si Mn P S Cr Ni Cu Ti Nb MoN Mg Al Sb Zr V Note 1 0.010 0.05 0.18 0.023 0.004 18.8 0.23 0.36 0.220.020 0.06 0.009 0.0003 0.028 — — — Example 2 0.003 0.10 0.15 0.0240.003 18.4 0.12 0.35 0.14 0.030 0.02 0.008 0.0001 0.030 — — — 3 0.0120.14 0.25 0.020 0.003 18.3 0.20 0.33 0.29 0.040 0.10 0.012 0.0002 0.025— — — 4 0.006 0.18 0.21 0.021 0.002 17.3 0.26 0.41 0.20 0.040 0.08 0.0090.0004 0.031 — — — 5 0.003 0.12 0.13 0.025 0.004 18.4 0.13 0.40 0.150.020 0.05 0.007 0.0003 0.150 — — — 6 0.008 0.08 0.19 0.026 0.005 18.20.15 0.34 0.19 0.010 0.07 0.008 0.0002 0.240 — — 0.20 7 0.007 0.21 0.330.019 0.002 18.0 0.27 0.50 0.24 0.030 0.08 0.012 0.0002 0.024 0.110 0.12— 8 0.014 0.29 0.22 0.022 0.003 17.9 0.11 0.31 0.21 0.020 0.03 0.0050.0003 0.160 0.006 — — 33 0.005 0.15 0.15 0.024 0.004 18.1 0.22 0.370.20 0.030 0.04 0.007 0.0004 — — — — 9 0.009 0.11 0.19 0.025 0.002 16.20.20 0.41 0.27 0.020 0.09 0.008 0.0003 0.026 — — — Comparative 10 0.0050.23 0.22 0.029 0.003 19.4 0.17 0.34 0.34 0.030 0.11 0.010 0.0003 0.210— — — Example 11 0.012 0.18 0.20 0.024 0.004 18.2 0.21 0.38 0.07 0.0200.13 0.015 0.0002 0.032 — 0.05 — 12 0.005 0.34 0.21 0.020 0.002 18.50.24 0.32 0.29 0.060 0.06 0.013 0.0004 0.110 — — 0.10 Notation: Underline indicates a value out of the range of the present invention.

Firstly, the surface quality of the obtained cold-rolled, annealed andpickled steel sheets was evaluated by a visual inspection.

Subsequently, using the obtained cold-rolled, annealed and pickled steelsheets, two kinds of samples for evaluating the corrosion resistance ofthe base steel sheet were prepared. One was a sample in a pickled stateobtained from the as pickled steel sheet after descaling and another wasa polished sample obtained by polishing the surface of the as pickledsteel sheet with a #600 emery paper.

Moreover, using the same steel sheets, a test on the welded part formedby TIG welding was carried out. In this test, two pieces cut out of eachsteel sheet were welded by TIG welding and polished with a #600 emerypaper for evaluating the corrosion resistance in the welded part of samesteels.

In addition, using the same steel sheets, a TIG welding test with aSUS304 as a different steel was carried out. In this test, a piece cutout of each steel sheet and a piece of SUS304 sheet having a thicknessof 1.0 mm were welded by TIG welding for evaluating the corrosionresistance in the welded part of different steels. The surface of thewelded part was polished with a #600 emery paper. The conditions ofwelding of the same steels and different steels will be described below.Welding current was controlled so that the width of the back bead was 3mm or more. The surface on the back bead side was evaluated.

Welding voltage: 10 V

Welding current: from 90 A to 110 A

Welding speed: 600 mm/min

Electrode: tungsten electrode of 1.6 mm

Shield gas: front bead side Ar 20 L/min,

-   -   back bead side Ar 20 L/min

Using the two kind of samples (sample in a pickled state and sample in apolished state), the welded part of same steels and the welded part ofdifferent steels, a neutral salt spray cyclic corrosion test (CCT) wascarried out in accordance with JIS H 8502 (1999). In the CCT, a cycle inwhich spraying a solution containing NaCl in a concentration of 5 mass %(35° C., 2 hours), drying (60° C., 4 hours, relative humidity: from 20%to 30%) and wetting (40° C., 2 hours, relative humidity: 95% or more)were performed in this order was repeated for 15 cycles. The obtainedresults are given in Table 2. Here, the evaluation standards of thetests will be described hereafter.

(a) Surface appearance after cold rolling, annealing and pickling:evaluated by means of the ratio of the length of portions in whichsurface defects (scab, pin hole, linear scab, stringer flaw caused bytitanium nitrides, abnormal color of white streak) were found to thetotal length of the sample. The surface appearance was evaluated in thefollowing way; └ indicates the case where a defect ratio is less than5%, ◯ indicates the case where a defect ratio is 5% or more and lessthan 10%, Δ indicates the case where a defect ratio is 10% or more andless than 20% and x indicates the case where a defect ratio is 20% ormore, where └ and ◯ indicate satisfactory cases and Δ and x indicateunsatisfactory cases.

(b) Corrosion resistance by CCT of the samples in a pickled state and ina polished state with a #600 emery paper: evaluated by means of an areain which rust occurred after the 15 cycle test. The result of the CCTwas evaluated in the following way; └ indicates the case where a rustarea ratio is less than 10%, ◯ indicates the case where a rust arearatio is 10% or more and less than 20%, Δ indicates the case where arust area ratio is 20% or more and less than 30% and x indicates thecase where a rust area ratio is 30% or more, where └ and ◯ indicatesatisfactory cases and Δ and x indicate unsatisfactory cases.

(c) Corrosion resistance by CCT of the welded part of same steels:evaluated by means of a rust area ratio after 15 cycles of the CCT whichwas performed on the samples obtained by TIG butt welding the samples ofsame steels and by eliminating the temper color of the welded part witha #600 emery paper. The results of the corrosion resistance wasevaluated in the following way; └ indicates the case where a rust arearatio is less than 10%, ◯ indicates the case where a rust area ratio is10% or more and less than 20%, Δ indicates the case where a rust arearatio is 20% or more and less than 30% and x indicates the case where arust area ratio is 30% or more, where └ and ◯ indicate satisfactorycases and Δ and x indicate unsatisfactory cases.

(d) Corrosion resistance by CCT of the welded part of different steels:evaluated by means of a rust area ratio after 15 cycles of the CCT whichwas performed on the samples obtained by TIG butt welding each sampleand SUS304 and by eliminating the temper color of the welded part with#600 emery paper. The results of the corrosion resistance was evaluatedin the following way; └ indicates the case where a rust area ratio isless than 10%, ◯ indicates the case where a rust area ratio is 10% ormore and less than 20%, Δ indicates the case where a rust area ratio is20% or more and less than 30% and x indicates the case where a rust arearatio is 30% or more, where └ and ◯ indicate satisfactory cases and Δand x indicate unsatisfactory cases.

TABLE 2 Corrosion Corrosion Resistance of Resistance of CorrosionCorrosion Cold-Rolled, Cold-Rolled, Resistance in Resistance in SurfaceQuality Annealed Annealed and Weld Zone Weld Zone of Cold-Rolled, andPickled Pickled Sheet of Same of Different Annealed and Sheet (Pickled(Polished State) Steel Grades Steel Grades No. Pickled Sheet State) inCCT in CCT in CCT in CCT Note Note 1 ⊚ ⊚ ⊚ ⊚ ◯ — Example 2 ⊚ ⊚ ⊚ ⊚ ◯ — 3◯ ⊚ ⊚ ⊚ ◯ — 4 ⊚ ◯ ⊚ ⊚ ◯ — 5 ⊚ ⊚ ⊚ ⊚ ◯ — 6 ⊚ ⊚ ⊚ ⊚ ⊚ — 7 ⊚ ⊚ ⊚ ⊚ ⊚ — 8 ◯⊚ ⊚ ⊚ ⊚ — 33 ⊚ ⊚ ⊚ ⊚ ◯ — 9 ⊚ X X X X — Comparative 10 ◯ ⊚ ⊚ ◯ X —Example 11 ⊚ ◯ ◯ X X — 12 Δ Δ ⊚ ⊚ ◯ Decreased Ductility Residual ScaleNotation: ⊚, ◯; satisfactory, Δ, X; unsatisfactory

Examples No. 1 through No. 8 and No. 33 having a chemical composition inthe preferred range according to the present invention were excellent incorrosion resistance and surface quality in all evaluation items. On theother hand, comparative example No. 9 having low Cr content of 16.2% waspoor in corrosion resistance, as indicated by its large rust areas.

Comparative example No. 10 having high Cr content of 19.4% was poor incorrosion resistance, as indicated by its large rust area in the weldedpart of different steels. This is thought to be because a martensitephase is not formed in the welded part of different steels due to thelarge content of Cr which is a ferrite former element.

Comparative example No. 11 having low Ti content of 0.07% was poor incorrosion resistance, as indicated by its large rust area in the weldedpart of different steels.

In the case of comparative example No. 12 having Si content and Nbcontent more than the preferred range according to the presentinvention, some residual scale was found on the surface of the basesteel sheet, and it was poor in corrosion resistance after cold rolling,annealing and pickling.

Example 2

Steels having a chemical composition of examples No. 13 through No. 18and comparative examples No. 19 through No. 22 given in Table 3 weresmelted using a VOD (Vacuum Oxygen Decarburization) having a capacity of150 ton and cast into slabs by continuous casting. These slabs wereheated at a temperature of 1150° C. and hot-rolled into hot-rolled steelsheet coil having a thickness of 4.0 mm. Then, these hot-rolled steelsheet coils were subjected to annealing in an atmosphere of a coke ovengas having an air ratio of 1.3 at a temperature of 950° C. for aduration of from 1 minute to 5 minutes. These hot-rolled and annealedsteel sheet coils were shot blasted with iron beads, descaled bypickling in which the steel sheet coils were dipped in a sulfuric acidsolution containing sulfuric acid in a concentration of 20 mass % at atemperature of 80° C. for a duration of 120 seconds and then in a mixedacid solution containing nitric acid in a concentration of 15 mass % andhydrofluoric acid in a concentration of 3 mass % at a temperature of 55°C. for a duration of 60 seconds. And these hot-rolled and annealed steelsheets coils were cold-rolled into cold-rolled steel sheet coils havinga thickness of 1.0 mm, subjected to annealing in a furnace in anatmosphere of a coke oven gas having an air ratio of 1.3 at atemperature of 900° C. for a duration of 2 minutes, electrolyticdescaling with the steel sheet being a positive electrode for threetimes in a solution containing NaSO₄ in a concentration of 20 mass % ata temperature of 80° C. with a current of 3 A/dm² for a duration of 10seconds, and descaling in a mixed acid solution containing nitric acidin a concentration of 5 mass % and hydrofluoric acid in a concentrationof 3 mass % at a temperature of 55° C. for a duration of 30 seconds inorder to obtain cold-rolled, annealed and pickled steel sheets.

TABLE 3 Chemical Composition (mass %) No. C Si Mn P S Cr Ni Cu Ti Nb 130.009 0.06 0.21 0.024 0.006 18.6 0.20 0.34 0.21 0.020 14 0.005 0.11 0.230.025 0.003 18.4 0.12 0.35 0.16 0.030 15 0.006 0.13 0.19 0.023 0.00817.9 0.19 0.46 0.23 0.040 16 0.006 0.10 0.22 0.025 0.004 18.5 0.15 0.340.17 0.030 17 0.010 0.28 0.20 0.019 0.003 18.3 0.23 0.36 0.37 0.020 180.004 0.19 0.33 0.030 0.005 18.2 0.24 0.31 0.20 0.030 19 0.007 0.15 0.190.023 0.003 18.2 0.18 0.34 0.24 0.020 20 0.004 0.26 0.24 0.029 0.00419.5 0.23 0.32 0.33 0.020 21 0.011 0.33 0.20 0.021 0.006 17.8 0.20 0.370.37 0.003 22 0.005 0.45 0.23 0.019 0.004 18.4 0.14 0.41 0.20 0.010Chemical Composition (mass %) No. Mo N Mg Al Sb Zr V Note 13 0.05 0.0080.0002 0.029 — — — Example 14 0.07 0.008 0.0003 0.030 — — — 15 0.080.012 0.0004 0.027 — — — 16 0.12 0.009 0.0004 0.140 — — — 17 0.06 0.0140.0003 0.028 — — 0.10 18 0.08 0.010 0.0002 0.031 0.100 0.12 — 19 0.400.010 0.0004 0.025 — — — Comparative 20 0.15 0.008 0.0001 0.026 — — —Example 21 0.08 0.015 0.0003 0.032 — — — 22 0.09 0.008 0.0010 0.200 — —0.20 Notation: Under line indicates a value out of the range of thepresent invention.

Firstly, the surface quality of the cold-rolled, annealed and pickledsteel sheets obtained as described above was evaluated by a visualinspection.

Subsequently, samples of the base steel sheet, the welded part of samesteels and the welded part of different steels were prepared in mannerssimilar to the ways described in Example 1, a neutral salt spray cycliccorrosion test was carried out in accordance with JIS H 8502 (1999) aswas done in Example 1 and corrosion resistance was evaluated. Theobtained results are given in Table 4. Here, the evaluation standards ofthe tests were similar to those in Example 1.

TABLE 4 Corrosion Corrosion Corrosion Corrosion Resistance of Cold-Resistance of Cold- Resistance Resistance in Surface Quality Rolled,Annealed Rolled, Annealed in Weld Weld Zone of Cold-Rolled, and PickledSheet and Pickled Sheet Zone of Same of Different Annealed and (PickledState) in (Polished State) in Steel Grades Steel Grades No. PickledSheet CCT CCT in CCT in CCT Note Note 13 ⊚ ⊚ ⊚ ⊚ ◯ — Example 14 ⊚ ⊚ ⊚ ⊚◯ — 15 ⊚ ⊚ ⊚ ⊚ ◯ — 16 ⊚ ⊚ ⊚ ⊚ ◯ — 17 ◯ ⊚ ⊚ ⊚ ⊚ — 18 ⊚ ⊚ ⊚ ⊚ ⊚ — 19 ⊚ ⊚ ⊚⊚ X — 20 ◯ ⊚ ⊚ ⊚ X — Comparative 21 X Δ ⊚ ⊚ ◯ — Example 22 X X ⊚ ⊚ ◯ —Notation: ⊚, ◯; satisfactory, Δ, X; unsatisfactory

Examples No. 13 through No. 18 having a chemical composition in thepreferred range according to the present invention were excellent incorrosion resistance and surface quality in all evaluation items.

On the other hand, since comparative example No. 19 had Mo content of0.40% more than the preferred range according to the present invention,and since comparative example No. 20 had Cr content of 19.5% more thanthe preferred range according to the present invention, both were poorin corrosion resistance, as indicated by their large rust area in thewelded part of different steels. This is thought to be because amartensite phase is not formed in the welded part of different steelsdue to the large contents of Mo and Cr, which are ferrite formerelements.

In addition, since comparative example No. 21 had Si content of 0.33%and Nb content of 0.003% out of the preferred range according to thepresent invention, and since comparative example No. 22 had Si contentof 0.45% and Mg content of 0.0010% more than the preferred rangeaccording to the present invention, some residual scale was found, andboth were poor in corrosion resistance after cold rolling, annealing andpickling.

Example 3

Steels having a chemical composition of examples No. 23 through No. 28and comparative examples No. 29 through No. 32 given in Table 5 weresmelted using a small vacuum melting furnace having a capacity of 50 kg.These ingots of these steels were heated at a temperature of 1150° C. ina furnace under an Ar gas purge and hot-rolled into hot-rolled steelsheets having a thickness of 4.0 mm.

Then, these hot-rolled sheets were subjected to annealing in air at atemperature of 950° C. for a duration of 1 minute, followed by a surfacetreatment using shot blasting with glass beads, and descaling bypickling in which the steel sheets were dipped in a sulfuric acidsolution containing sulfuric acid in a concentration of 20 mass % at atemperature of 80° C. for a duration of 120 seconds and then in a mixedacid solution containing nitric acid in a concentration of 15 mass % andhydrofluoric acid in a concentration of 3 mass % at a temperature of 55°C. for a duration of 60 seconds.

Then, the hot-rolled steel sheets were cold-rolled into cold-rolledsteel sheets having a thickness of 1.0 mm, subjected to annealing in areducing atmosphere (H₂: 5 volume %, N₂: 95 volume %, dewpoint: −40° C.)at a temperature of 900° C. for a duration of 1 minute and madecold-rolled and annealed steel sheets. These cold-rolled and annealedsteel sheets were subjected to descaling using electrolysis (10 A/dm²for 2 seconds) with the steel sheet being a positive electrode for twotimes in a solution containing nitric acid in a concentration of 15 mass% and hydrochloric acid in an concentration of 0.5 mass % at atemperature of 50° C. in order to have cold-rolled, annealed and pickledsteel sheets.

TABLE 5 Chemical Composition (mass %) No. C Si Mn P S Cr Ni Cu Ti Nb MoN Mg Al Sb Zr V Note 23 0.010 0.18 0.28 0.024 0.004 18.7 0.15 0.36 0.230.020 0.11 0.012 0.0004 0.028 — — — Example 24 0.005 0.11 0.21 0.0250.002 18.4 0.16 0.34 0.16 0.030 0.07 0.008 0.0002 0.030 — — — 25 0.0060.09 0.19 0.023 0.003 18.5 0.20 0.35 0.17 0.030 0.09 0.007 0.0003 0.150— — — 26 0.009 0.06 0.23 0.024 0.005 17.3 0.13 0.32 0.20 0.010 0.130.010 0.0002 0.027 0.180 0.20 — 27 0.013 0.28 0.20 0.025 0.003 18.2 0.180.40 0.35 0.040 0.06 0.013 0.0003 0.190 — — 0.10 28 0.003 0.12 0.230.024 0.004 17.9 0.21 0.37 0.12 0.030 0.05 0.008 0.0004 0.160 0.050 —0.40 29 0.010 0.18 0.20 0.029 0.005 16.7 0.15 0.43 0.27 0.020 0.11 0.0100.0003 — — — — Comparative 30 0.005 0.20 0.24 0.027 0.001 19.7 0.23 0.310.32 0.020 0.15 0.008 0.0001 0.210 — — — Example 31 0.012 0.36 0.210.024 0.003 18.0 0.24 0.36 0.25 0.020 0.40 0.011 0.0004 — — — — 32 0.0050.50 0.25 0.026 0.004 18.4 0.18 0.32 0.23 0.100 0.09 0.008 0.0003 — — —0.20 Notation: Under line indicates a value out of the range of thepresent invention.

Firstly, the surface quality of the obtained cold-rolled, annealed andpickled steel sheets was evaluated by a visual inspection.

Subsequently, using the cold-rolled, annealed and pickled steel sheets,two kinds of samples for evaluating the corrosion resistance of the basesteel sheet were prepared as was done in Example 1. One was a sample ina pickled state obtained from the as pickled steel sheet after descalingand another was a polished sample obtained by polishing the surface ofthe as pickled steel sheet with a #600 emery paper.

In preparation of samples of welded part of same steels and welded partof different steels using a SUS304, considering the case where gasshield is not sufficiently effective during TIG welding, welding testwas carried out using a shield gas of Ar+20 volume % N₂ on both thefront and back bead sides.

The conditions of welding will be described below. The surface on theback bead side was evaluated.

Welding voltage: 10 V

Welding current: from 90 A to 110 A

Welding speed: 600 mm/min

Electrode: tungsten electrode of 1.6 mm

Shield gas: front bead side Ar+20 volume % N₂ 20 L/min,

-   -   back bead side Ar+20 volume % N₂ 20 L/min

Using the two kind of samples (sample in a pickled state and sample in apolished state), the welded part of same steels and the welded part ofdifferent steels, a neutral salt spray cyclic corrosion test (CCT) wascarried out in accordance with JIS H 8502 (1999). In the CCT, a cycle inwhich spraying a solution containing NaCl in a concentration of 5 mass %(35° C., 2 hours), drying (60° C., 4 hours, relative humidity: from 20%to 30%) and wetting (40° C., 2 hours, relative humidity: 95% or more)were performed in this order was repeated for 15 cycles. The obtainedresults are given in Table 6. Here, the evaluation standards weresimilar to those in Example 1.

TABLE 6 Corrosion Corrosion Corrosion Corrosion Resistance of Cold-Resistance of Cold- Resistance Resistance in Surface Quality Rolled,Annealed Rolled, Annealed in Weld Weld Zone of Cold-Rolled, and PickledSheet and Pickled Sheet Zone of Same of Different Annealed and (PickledState) (Polished State) Steel Grades Steel Grades No. Pickled Sheet inCCT in CCT in CCT in CCT Note Note 23 ⊚ ⊚ ⊚ ◯ ◯ — Example 24 ⊚ ⊚ ⊚ ◯ ◯ —25 ⊚ ⊚ ⊚ ⊚ ⊚ — 26 ◯ ◯ ⊚ ⊚ ⊚ — 27 ⊚ ⊚ ⊚ ⊚ ⊚ — 28 ⊚ ⊚ ⊚ ⊚ ⊚ — 29 ⊚ X X X X— Comparative 30 ⊚ ⊚ ◯ Δ X — Example 31 Δ Δ ◯ ◯ X — 32 X X ⊚ ◯ XDecreased Ductility Residual Scale Notation: ⊚, ◯; satisfactory, Δ, X;unsatisfactory

Examples No. 23 through No. 28 were excellent in corrosion resistanceand surface quality in all evaluation items. Examples No. 25 through 28to which Al, Sb Zr and V were added were significantly excellent incorrosion resistance even in the welded part of different steels using aSUS304.

On the other hand, since comparative example No. 29 had Cr content of16.7% less than the preferred range according to the present invention,it was poor in corrosion resistance, as indicated by its large rustarea.

In addition, since comparative example No. 30 had Cr content of 19.7%more than the preferred range according to the present invention, it waspoor in corrosion resistance, as indicated by its large rust area in thewelded part with different steels. This is because a martensite phase isnot formed in the welded part of different steels due to the largecontent of Cr, which is a ferrite former element.

In addition, comparative example No. 31 had Si content of 0.36% and Mocontent of 0.40% more than the preferred range according to the presentinvention, some residual scale was found on the surface of the basesteel sheet. And it was poor not only in corrosion resistance after coldrolling, annealing and pickling, but also in corrosion resistance in thewelded part of different steels using a SUS304 in which, in particular,gas shield was not sufficient.

Moreover, comparative example No. 32 had Si content of 0.50% and Nbcontent of 0.10% more than the preferred range according to the presentinvention, some residual scale was found on the surface of the basesteel sheet, and it was poor in corrosion resistance after cold rolling,annealing and pickling.

As described above, it has been clarified that, according to the presentinvention, a ferritic stainless steel excellent in corrosion resistanceof base steel sheet, corrosion resistance in the welded part of samesteels, corrosion resistance in the welded part of different steelsusing a SUS304 and the surface quality of cold-rolled, annealed andpickled sheet can be obtained without grinding the surface ofhot-rolled, annealed and pickled steel sheet.

The ferritic stainless steel according to the present invention can bepreferably used as a material for parts which need corrosion resistancesuch as kitchen and house wares, architectural interior and exterior,building parts, the interior of an elevator and an escalator, electricappliances and automobile parts.

1. A ferritic stainless steel having a chemical composition consistingof, by mass %, C: 0.003% or more and 0.015% or less, Si: 0.05% or moreand 0.30% or less, Mn: 0.10% or more and 0.35% or less, P: 0.06% orless, S: 0.02% or less, Cr: 17.0% or more and 19.0% or less, Ni: morethan 0.10% and 0.30% or less, Ti: 0.10% or more and 0.40% or less, Nb:0.005% or more and less than 0.050%, Mo: less than 0.20%, N: 0.005% ormore and 0.015% or less, Cu: 0.30% or more and 0.50% or less, Mg: lessthan 0.0005% and the balance being Fe and inevitable impurities.
 2. Theferritic stainless steel according to claim 1, the steel having achemical composition further containing, by mass %, Al: 0.02% or moreand 0.50% or less.
 3. The ferritic stainless steel according to claim 2,the steel having a chemical composition containing, by mass %, Al: 0.10%or more and 0.50% or less.
 4. The ferritic stainless steel according toclaim 1, the steel having a chemical composition further containing, bymass %, Sb: 0.005% or more and 0.300% or less.
 5. The ferritic stainlesssteel according to claim 1, the steel having a chemical compositionfurther containing, by mass %, at least one of Zr: 0.05% or more and0.60% or less and V: 0.02% or more and 0.50% or less.
 6. The ferriticstainless steel according to claim 2, the steel having a chemicalcomposition further containing, by mass %, Sb: 0.005% or more and 0.300%or less.
 7. The ferritic stainless steel according to claim 3, the steelhaving a chemical composition further containing, by mass %, Sb: 0.005%or more and 0.300% or less.
 8. The ferritic stainless steel according toclaim 2, the steel having a chemical composition further containing, bymass %, at least one of Zr: 0.05% or more and 0.60% or less and V: 0.02%or more and 0.50% or less.
 9. The ferritic stainless steel according toclaim 3, the steel having a chemical composition further containing, bymass %, at least one of Zr: 0.05% or more and 0.60% or less and V: 0.02%or more and 0.50% or less.
 10. The ferritic stainless steel according toclaim 4, the steel having a chemical composition further containing, bymass %, at least one of Zr: 0.05% or more and 0.60% or less and V: 0.02%or more and 0.50% or less.